DRC rule to check Density of a circle

Hi,

How would you write a Calibre DRC rule that checks the density of a circle? I've tried the following:

CUP.14_M1 { @ Minimum density of Metal1 area under copper pillar >= 50%

   DENSITY Metal1 < (50/100) INSIDE OF LAYER Pillar RDB CUP.14_M1.rdb

}

where both Pillar and Metal1 are drawn as circles. However, this will take the bounding box of the layer and this results in incorrect flagging. Any help is appreciated.